CHARLOTTE, N.C. – Just as the city of Charlotte, N.C., gears up to host the Democratic National Convention, an atheist group is mounting a billboard campaign attacking the religious faiths of President Obama and GOP challenger Mitt Romney.

The signs, paid for by American Atheists Incorporated (AAI) and appearing along keylocal highways, include messages such as: "Christianity: Sadistic God, Useless Savior" and "Mormonism: Magic Underwear, Baptizes Dead People, Big Money, Big Bigotry." AAI President David Silverman told FoxNews.com the signs are aimed at keeping religion and politics separate as the convention gets under way Sept. 3.

Other faiths, including Judaism and Islam, were spared because the billboards are designed to attack GOP nominee Mitt Romney, a Mormon, and President Obama, a Christian, according to AAI. They will remain up for a month, and cost Silverman's group $15,000.

"We chose Christianity and Mormonism for these billboards because those are the religious faiths of the two presumed candidates for president," said Amanda Knief, managing director for American Atheists "The president of the United States is in a position to make life-changing decisions for all Americans. We believe it is perfectly reasonable to ask whether each candidate will choose to follow his religious faith or the U.S. Constitution when making those decisions."

Kevin Madrzykowski, general manager of the Charlotte office of Adams Outdoor Advertising, which rented billboard space to AAI, said his company backs the group's right to get its message out.

"The upcoming Democratic National Convention will bring to Charlotte people of varying viewpoints and a diversity of opinion," Madrzykowski said in a statement. "The ability to express one's opinion is a right and a privilege at the core of a democratic society."

The anti-Mormon billboard was originally planned for Tampa, where Mitt Romney will accept his nomination for president at the Republican National Convention. But private billboard companies in Tampa refused to put up the ad. Clear Channel Outdoor spokesman Jim Cullinan told FoxNews.com the American Atheists' campaign was not appropriate.

“We don’t accept attack ads, we just don’t do that,” Cullinan said. "So we tried to work with them to say 'Is there a way to change your creative so it’s not an attack ad?' And they decided not to.

"We tried to work with them and they were clearly trying to make a point, and we simply aren’t going to put that kind of ad up on our billboards ever.”

Silverman said the Florida companies' refusal to post billboards attacking peoples' beliefs was a case of bigotry against atheists.

“We are very happy that we found a company that allowed us to express our freedom of speech, our opinions [in Charlotte],” said Silverman. “We are also very dismayed at the bigotry that we received in Tampa when we weren’t allowed to post our views.”

The First Amendment and freedom of speech protect American Atheists, making the billboards perfectly legal. FoxNews.com reached out to representatives for the Democratic National Convention for comment on the billboards but did not hear back.

Should religion play a role in the electorates vote for President? “We are not a Christian nation; we have never been a Christian nation and we never will be,” said Silverman.

But some argue that religion is at the core of America. “All you have to do is go to Washington and look at all the Scripture quotations all over the buildings in Washington, D.C.,” said Matthews.

The Republican National Convention will be in Tampa August 27 - 30. The Democratic National Convention will be in Charlotte September 3 - 6.